In recent years, to cope with air pollution or global warming, reducing the amount of carbon dioxide is strongly desired. In the automobile industry, reducing the amount of carbon dioxide emission by introducing an electric vehicle (EV) or a hybrid electric vehicle (HEV) is attracting attention, and thus development of an electric device like a secondary battery for driving a motor, which plays a key role in commercialization, is actively under progress.
Compared to a consumer lithium ion secondary battery for a cellular phone, a notebook computer or the like, the motor-driving secondary battery is required to have very high output characteristics and very high energy. Thus, among the all batteries, the focus is made on a lithium ion secondary battery having the highest theoretical energy, and a rapid development is now under progress.
In general, a lithium ion secondary battery has a configuration that a positive electrode in which a positive electrode active material is applied to both surfaces of a positive electrode current collector with the use of a binder and a negative electrode in which a negative electrode active material is applied to both surfaces of a negative electrode current collector with the use of a binder are connected to each other via an electrolyte layer and accommodated in a battery case.
Conventionally, a carbon.graphite-based material is used for a negative electrode of a lithium ion secondary battery, in terms of the charge and discharge cycle life and the cost advantage. However, the carbon.graphite-based negative electrode material performs charge and discharge operation by absorption and desorption of lithium ions to and from graphite crystals. As such, there is a disadvantage that the charge and discharge capacity equal to or higher than 372 mAh/g, which is the theoretical capacity obtained by LiC6 as a compound with maximum Li introduction, cannot be obtained. Therefore, it is difficult for a carbon.graphite-based negative electrode material to secure a satisfactory level of capacity and energy density for practical use in a vehicle.
On the other hand, a material capable of forming an alloy with Li is expected as a negative electrode material for vehicle use due to the fact that the battery using this material improves in energy density compared to the battery suing a conventional carbon.graphite-based negative electrode material. For example, Si material exhibits absorption and desorption of lithium ions in an amount of 4.4 mol per mol as shown in the following reaction formula (A), and the theoretical capacity is 2100 mAh/g for Li22Si5 (=Li4.4Si). Furthermore, when calculated as a value per Si weight, it has initial capacity of 3200 mAh/g.Si+4.4Li++e−Li4.4Si  (A)
In the lithium ion secondary battery, however, a negative electrode using a material capable of forming an alloy with Li shows high degree of expansion and shrinkage during charge and discharge operation. For example, while the graphite material shows volume expansion of about 1.2 times by absorption of Li ions, the Si material exhibits a large volume change (about 4 times) by transition from amorphous to crystal phase during alloying of Si with Li. This results in a deterioration of cycle life of an electrode. Furthermore, the Si negative electrode active material has a trade-off relationship between capacity and cycle durability so that it is difficult to improve the cycle durability while securing the high capacity.
To solve those problems, a negative electrode active material for a lithium ion secondary battery containing an amorphous alloy with formula SixMyAlz has been suggested (for example, see JP-A-2009-517850 (WO 2007/064531 A1)). In the formula, x, y, and z represent atomic percentage values, x+y+z=100, x≥55, y<22, and z>0, and M is a metal consisting of at least one of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr, and Y. According to the invention described in JP-A-2009-517850 (WO 2007/064531 A1), it is disclosed in paragraph [0008] that it is possible to attain not only high capacity but also good cycle life by minimizing the content of metal M.
Although it is described in JP-A-2009-517850 (WO 2007/064531 A1) that a lithium ion secondary battery using a negative electrode active material, which contains an amorphous alloy with formula SixMyAlz, can exhibit good cycle durability, it cannot be said that the cycle durability is sufficient.